@Jadro, https://repl.it should allow you to run some kind of HTTP server code there and it exposes that to the public Internet. I have not tried Python with Flask, but I tried using Node.js in this way and it works. But your code should be visible to the public as well (using free account).

I also found it will not keep your server running forever, it will only start the VM and start your server code if someone tries to accessing it. So user may need to wait a bit longer initially if he/she accessing it when the server code is not running.